#include <iostream>
#include <string>
#include <stack>
using namespace std;
bool isOp(char* a)
{
    if(a == '+'||a == '-'||a == '*'||a == '/'||)
        return true;
    else
        return false;
}
string swap_brace(string& a)
{
    string string_out = "("+a<")";
    return string_out;
}
int main() {
    stack<char> op;
    stack<char> op_num;
    string exp[] = {"2", "1", "+", "3", "*"};
    for(auto i:exp)
    {
        if(!isOp(i))
            op_num.push(i);
        else {
            char op1[2];
            for(int i=0;i<2;i++)
            {
                op1[i] = op_num.top();
                op.pop();
            }
            op1[0]+op[1]

            op.pop();
            char op2 = op.top()
            op_stack.push()
        }

    }
    return 0;
}